Cicadas: the science behind the invasion
They’re back. And if they’re out in your neighborhood, they’re pretty hard to miss.
I’m talking about the periodical cicadas. In the past few weeks, they’ve emerged by the billions in states from Maryland to Georgia to Oklahoma.
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What’s your reaction?
“I love cicadas!” exclaimed 10-year-old Matt Covington. It’s a muggy afternoon in late May at the Kirkwood farmer’s market. “I just like holding them and putting them on the trees and stuff,” he explained. “They tickle when they crawl on you. It’s fun.”
Nineteen-year-old Lauren Pierce loves them…well…not so much. “Oh my gosh, they look like little devils, and I don’t like it!” Pierce said. “I think they’re stinking up my neighborhood. They’re rotting everywhere and they smell bad, and they’re really loud in some places, and…it’s kind of a nuisance.”
They are definitely really loud. Those are the males, singing to attract mates. Chris Hartley, an entomologist at the Sophia M. Sachs Butterfly House in Chesterfield, says they make that sound by vibrating an organ on the side of their thorax called a tymbal.
“Now a tymbal works a lot like the freshness seal that you get on top of a bottle of juice, the things that you click in and out,” Hartley explained. “Except they vibrate it really quickly and they actually use the wings to amplify and resonate the sound out, which is part of what makes it so loud.”
But these periodical cicadas have spent the past 13 years not making any noise at all. They’ve been underground, feeding on the juices of tree roots, as small brown juveniles, or nymphs.
“The nymphs are these strange kind of alien looking things that have no wings, very rudimentary legs that are only good for crawling and nothing else, and they kind of have these huge, claw-like front legs,” Hartley said.
Emerging as adults
When they’re ready to emerge, the nymphs tunnel their way to the soil surface in the middle of the night, leaving visible holes in the ground about a half-inch in diameter. They climb up the nearest bush or tree and molt for the last time, littering the foliage with their old brown casings.
When the adult cicadas first emerge in the darkness, they’re soft, pale white, and very vulnerable. It takes a couple of hours for their exoskeleton to darken, and their wings to unfold and dry to a translucent orange-gold.
The red-eyed adults are a little over an inch long and live for three to four weeks.
Renewing the 13-year cycle
Glenn Frei helps take care of the live insect collection at the Saint Louis Zoo. “A female will lay, throughout her lifespan, about 600 eggs,” Frei said. He says female cicadas use a sword-like organ called an ovipositor to slice into small tree branches and insert their eggs.
When theant-sized nymphs hatch six to ten weeks later, they drop to the ground, dig a burrow, and start the 13-year cycle all over again.
Cicadas aren’t locusts
The egg-bearing twigs often turn brown, droop, and die. But Frei says cicadas aren’t locusts – they don’t eat leaves or attack crops.
“They’re causing some minor damage, but overall, even with the massive numbers of them, they’re not going to do enough to kill a healthy full grown tree,” Frei explained. “Young saplings may get enough damage where it could kill them, just because they’re not old enough, but for the most part a healthy, older tree is going to do just fine.”
Frei says the number of adult periodical cicadas has already peaked in our area, and we can expect this year’s cohort to die off in the next couple weeks.